Bone staples have been in clinical use for decades. These important bone fixation devices have evolved from rigid stainless steel or cobalt-chromium U-shaped implants to staples that could be manipulated to compress two adjacent bone segments.
The early rigid staples were commonly hammered into bone where the more modern devices are implanted in drilled holes and use heat or mechanical means to cause the staple to change shape and pull together and in some designs compress the bony segments. Bone staple technology used to pull bone together includes: 1) staples that are bent by an instrument: BENDABLE STAPLES, 2) heat sensitive shape memory alloy staples: MEMORY STAPLES, and 3) mechanical elastic bone staples: ELASTIC STAPLES.
The staple embodiments of this invention have advantages over the prior art because it stores mechanical energy and imparts that energy to bone through shape change and predictable bone-to-bone compression. The staple embodiments of this invention pull together and compress bone to promote healing. The prior art implants may change shape or be caused to change shape but do not pull together and compress bone with a predicable amount of shape change and compression force.
Instruments, to implant staples into bone, complement the staple's method of action. BENDABLE STAPLES use pliers, forceps, and complex instruments to apply the bending force. MEMORY STAPLES must be kept cold for body temperature heating or use an electrical resistive heating instrument to transition their crystalline structure from martensitic to austenitic. Prior ELASTIC STAPLES use pliers, hooks, forceps, and complex instruments to stretch and hold their shape while being implanted. These designs cause the surgeon to need to manipulate the implant while trying to implant it in bone. Thus these implants were difficult to implant or required complex expensive instruments thus impeding their use.
As will be clear in the following detailed description of the prior art, the embodiments illustrated of the subject invention overcome the prior art deficiencies in ease of use, manufacturing, mode of operation, strength, cost and allows hospital procedures that limit disease transmission.
Bendable Staples
Bendable staple designs use an instrument to bend the staple to facilitate placement, retention and bone movement. These designs can be bent to pull the bone together but partially spring open and provide no bone-to-bone compression.
Murray, in U.S. Pat. No. 3,960,147 uses pliers to squeeze the bridge of a staple to toe it in to enhance fixation. Weaver, in U.S. Pat. No. 4,444,181 uses a dual bridge staple and pliers to decrease the distance between the legs of a staple when the dual bridge is squeezed together. Garner, in U.S. Pat. No. 4,841,960 uses pliers to squeeze the bridge of a staple to bring the legs together.
Groiso, in U.S. Pat. Nos. 5,853,414, 5,449,359 and 7,635,367 uses pliers to bend a dual bridge titanium alloy or stainless steel staple, widening the bridge to shorten the distance between the legs and narrowing the bridge to lengthen the leg distance. Groiso calls this bending or permanent deformation “elastic” behavior when in fact this bending is mostly plastic deformation. Stainless steel and titanium alloy's elastic behavior is characterized by the 2% offset yield strength. Thus when bent with pliers or forceps the material undergoes both elastic (recoverable) and plastic (permanent) deformation. This elastic behavior under strain causes the staple to partially return to its pre-bent shape. The staple legs thus partially “spring back” and thus this type of staple does not cause bone segment compression once the pliers are no longer bending the staple. Groiso, in published continuation application Ser. No. 11/197,174 adds nitinol and shape memory features to his staple.
Hardengen, in published Application Ser. No. 10/940,396 also uses pliers and in its continuation application Ser. No. 12/582,210 Hardengen describes shape memory metal to widen the dual bridge screw plate of its parent application. Hardengen's invention is embodied in the Charlotte Staple and described in the Wright Medical, Charlotte Foot and Ankle Fixation System, page 4 and 6 document number SO 040-105 Rev. 04.06
These bendable implants bring the bone together, allow it to partially spring apart and provide no compression once the instrument is removed. They store no mechanical energy. They cannot continue to change shape to pull the bone together if a gap occurs during healing. This gap can result in delayed or non-healing. Consequently, with this impaired healing observation clinical demand for this type of bone staple has decreased. The embodiments of the subject invention of this patent overcomes the deficiencies of the bendable staples by not requiring manipulation of the implant and by storing shape changing elastic mechanical energy that continuously applies force to bone to pull it together and compress.
Memory Staples
Memory staples fabricated from the nickel-titanium alloy, nitinol, exhibit a shape memory effect when heated within their martensitic and austenitic microstructure temperature transition range. A U-shaped implant can be fabricated so that it returns to a predetermined final shape. Traditionally these implants have parallel legs and then when heated the legs change shape at the corners of the U-shaped bridge to bring the tips of the legs together so as to lock in bone and in some designs create bony compression. The bridge of these staples often have a geometry capable of changing shape so it can be shortened to provide further bony compression. These heat sensitive implants can have their shape change temperature varied by changes in their composition, residual stress in the material and heat treatment.
Mai, U.S. Pat. No. 5,246,443 used a martensitic to austenitic transition temperature of 10° C. to 15° C. and described a number of bone staples and plates and relied on body heat to initiate the transformation. Mai, in U.S. Pat. No. 5,474,557 presented a temperature transition range of −20° C. to 70° C. of which temperatures over 37° C. exceed body temperature and further described other staples, plates and clips. Bertolet, in U.S. Pat. No. 5,779,707 introduced shape changing holes and slots to shorten the bridge section of plates, staples and clips but again used martensitic to austenitic transformations at body temperature to affect their shape. Ogilvie, U.S. Pat. Nos. 6,325,805 and 6,773,437 expanded the use of body temperature staples for correction of spinal deformity.
These heat sensitive staples that rely on microstructure transition are problematic because during implantation the staple is in its mechanically soft martensitic state and commonly deform inappropriately with the impaction of surgical placement. Furthermore, during shipping, costly strategies must be implemented to keep environmental heating from causing the staple to change shape prior to implantation. Finally, a heating strategy must be used to activate the implant.
Originally heat sensitive nitinol staples were activated with the temperature of the human body, approximately 37° C. This strategy and implant formulation caused critical issues by changing shape and applying bone fixation forces only after the surgical wound had been closed and allowed to warm to normal body temperature. This post surgery shape change was reported to cause deformity and fracture. This style of nitinol staple was further inconvenient in its use because the transition temperature began at below room temperature thus these implants were changing shape while being implanted by the surgeon. Strategies such as keeping the staple on dry ice were used to partially overcome this issue but it added cost and the surgeon had to work quickly in procedures where deliberate and detailed technique was required.
The body temperature nitinol staples are further described in Biopro, Inc.'s Memory Staple Brochure, and Depuy Inc.'s Memory Staple Brochure. The review of the prior art patent, technical and sales literature it is clear that the cost, inconvenience and risk of use of body temperature staples have impeded clinical adoption due to complications.
Staples that changed shape at temperatures above body temperature were developed to avoid the implant changing shape during surgery and to provide shape change and force control of the implant. Fox, U.S. Pat. No. 7,240,677 used a controlled amount of electrical current passed through the metal to resistively heat staples above body temperature to convert the martensitic crystalline structure to austenitic. Fox, U.S. Pat. No. 7,240,677 set the transition temperature of the implant and the resistive current heating level so that this elevated temperature implant was below the heat level of tissue injury. This invention is further illustrated in the BioMedical Enterprises, Inc., BME_OSStaple_sell_sheet_B. Flot, U.S. Pat. Nos. 6,323,461 and 6,268,589 used electrical current to heat the staple but had no ability to control the extent of staple shape change.
Though Fox's, U.S. Pat. No. 7,240,677 elevated temperature staple heating strategies have seen extensive clinical use, this style and the body temperature heated implants are deficient due to variation in bone fixation force due to environmental heating or cooling and are soft in their mechanical properties during implantation. These issues and the requirement to have dry ice or an electrical bipolar heating unit have limited the clinical adoption of elevated temperature staples.
The embodiments of the subject invention of this patent overcomes the deficiencies of the memory staples such as 1) requiring heating or cooling, 2) having a temperature dependent fixation force, 3) requiring ancillary equipment to manipulate the implant, 4) being implanted in the soft martensitic phase, 5) requiring an expensive multiple step manufacturing process to set both the staple shape and transition temperatures, and 6) others that become more clear in the review of the embodiments of the subject invention.
Elastic Staples
Mohr, U.S. Pat. No. 3,939,828 first introduced the use of elastic properties of stainless steel for a bone staple. This invention the Osteoclasp™ was an S-shaped bridge staple with convergent legs. (A staple has a “convergent” shape when the legs of the staple are in a convergent orientation, as opposed to a substantially parallel orientation or a divergent orientation). In use, one leg was placed in an angled drill hole and the other pulled with a hook until it could be inserted in a second drill hole. The elastic spring-back of the stainless steel pulled the bone together and caused bone-to-bone compression. The legs are not manipulated to converge and compress, though Mohr's angled drill holes impede staple extrusion from bone. The clinical use of the Mohr staple has been long discontinued due to difficulty in stretching the bridge during placement and the frequency of having the staple unexpectedly released from the hook and spring from the surgical field.
Allen, in U.S. Pat. Nos. 6,348,054, 6,059,787 and 6,783,531 used a bowed bridge shaped staple and a complex instrument to pull the legs of the staple apart to straighten the bowed bridge while impacting the staple legs into bone. The elastic spring back of the bowed bridge staple pulled the bone together and caused bone-to-bone compression. Allen does not manipulate the legs and thus the parallel legs do not converge and resist extrusion from the bony drill holes. The cost of the instrument is high and no commercial embodiment of this invention is known.
Jervis, in U.S. Pat. Nos. 5,067,957 and 4,665,906 introduced the use of nitinol formulated to fully transition from stress induced martensite to austenite at body temperature for the fabrication of bone staples, plates and rods. Monassevitch, in U.S. Pat. No. 6,685,708 teaches the use of pliers or forceps on nitinol staples to plastically change the distance between the legs and allow the martensitic to austenitic crystalline structure of nitinol to move the legs back to the original distance once released. This invention requires the surgeon to change the shape of the staple during implantation, has high fixation force variation and does not provide a feature to impact the staple into bone. The shape recovery causes the staple bridge to shorten but does not angle the legs to resist extrusion from the bony drill holes. Monassevitch, claims a hand operated instrument for manipulating the staple and teaches that the staple must be cold and in its soft martensitic state so that the hand operated instrument has enough force to deform the staple. This is a sufficient deficiency because, the hand deforming is not precise, the staple must be sterile and made cold before deforming and the implant is soft when implanted and thus may bend with the impaction of placement in bone.
Memometal, Inc. sells an elastic staple, the EasyClip™. The EasyClip™ has a straight bridge and convergent legs. Pliers are used to pry the legs apart so that they can be inserted in predrilled holes. When the pliers release the staple legs they can swing in if the drill holes are loose or the bone is soft. The EasyClip™ cannot pull together and compress bone because the bridge is straight and constrained in the drill holes. This straight and rigid bridge defeats compression. The inward movement of the legs only tightens the legs in the holes to impede extrusion of the staple from the bone holes.
The simultaneous requirement for the surgeon to open the staple legs and insert the implant into bone is surgically difficult in many procedures, and for the other reasons noted above, have limited the clinical use of this implant. Memometal, Inc.'s Easy Clip Brochure further illustrates the deficiencies of these staple implants that are elastic and manipulated with pliers, complex instruments, forceps and hooks for stretching. The Easy Clip is described as having super elastic properties and there is no indication that the opening of the legs with pliers creates stress induced martensite in the staple to leg corners and certainly not in the straight bridge.
Though Jervis describes staples and many other medical implant applications the geometry of the staple is not described. Monassevitch presents a Z-shaped bridge that can be compressed into an S-shape but teaches away from legs angled in relation to the bridge and promotes a non-shape changing leg to bridge corner. The Easy Clip has a straight bridge and though its legs can deflect inwards to tighten in the hole this device cannot pull together and compress. This prior art stress induced martensite or super elastic implants have not taken advantage of the geometric leverage provided by the O-shaped or S-shaped bridge at contracting or lengthening or the bridge to leg corner to enhance the amount the staple can pull together and compress two structures. The prior art teaches elastic behavior but teach away from a staple geometry that creates optimal shape change and compression. Consequently, in use these implants have significant disadvantages compared to the embodiments of the subject invention.
The embodiments of the subject invention of this patent overcomes the deficiencies of the prior elastic staples such as 1) requiring the surgeon to stretch the staple to place it in bone, 2) designs that cannot contract their bridge, 3) requiring expensive ancillary equipment such as staple guns to manipulate the implant, 4) requiring the surgeon to change the staple shape with pliers, forceps or other hand operated instruments, 5) cooling of the implant prior to opening for placement, 6) designs that can not simultaneously provide in their bridge and legs geometric leverage to pull together and compress bone, and 6) others deficiencies that will become more clear in the review of the embodiments of the subject invention.
Instrument and Staple Implant Devices and Methods
Shapiro, U.S. Pat. No. 4,414,967 describes a pneumatic staple gun that combined with a staple cartridge violently impacted staples into bone. The staple's legs were divergent so that they pull bone together when inserted. (A staple has a “divergent” shape when the legs of the staple are in a divergent orientation, as opposed to a substantially parallel orientation or a conversion orientation). This implant did not change shape to pull together and compress bone. The instrument was complex, expensive and in aged porous bone sometimes caused bone fracture during staple insertion.
Assell, U.S. Pat. No. 4,527,726 and Bent, U.S. Pat. No. 4,540,110, as did Shapiro U.S. Pat. No. 4,414,967, both illustrated an automatic stapler that forces a staple down a channel with significant energy to impact this implant into bone. These staples do not store mechanical energy or change shape and thus the staples of his system cannot pull together and compress bone. The convenience of these systems is overcome by the high cost, complicated design of the staple gun, and difficulty in cleaning and sterilizing the stapler for repeated patient use.
McHarrie, U.S. Pat. No. 4,415,111 proposed a locator tube having a staple in a slot and a cooperating punch to push the staple from the tube into bone. McHarrie's invention cannot be used with shape changing staples because it does not constrain the staple legs from swinging in or the bridge from shortening. Consequently, the staples of this system do not change shape to pull together and compress bone. Pratt, U.S. Pat. No. 4,438,769, used a simple system to hold the staple bridge in a grasping driver that used a threaded coupler to lock the staple. This system supported the staple during hammer insertion into bone and through its geometry may urge bone together. The staple did not change shape to pull together and compress bone because the system required rigid staples to withstand the bone impaction forces.
The foregoing discussion illustrates the deficiencies of the prior art and the lack of a simple shape changing staple, instrument for its implantation and method of use consistent with the demands of surgery. In the discussion of the embodiments of the subject invention its benefits will be realized as a simple, reliable, low cost solution to present an elastic energy storing shape changing staple to bone and releasing the staple so that it can pull together and compress bone even in the presence of gaps that can form during bone healing.